Inverters are used to convert direct current, for example generated by a photovoltaic generator of a photovoltaic installation, into an alternating current suitable for feeding into a power supply grid. In view of the increasing spread of regenerative power generation plants, in particular photovoltaic installations, the requirements of the power supply companies with respect to parameters such as the current which is fed in are increasing. On the part of the operators of power supply grids there is often the requirement, as is specified in the so-called grid code, that, in the event of grid disturbances, for example in the event of voltage dips, regenerative power generation plants must be able to ride through the grid disturbance (fault ride-through—FRT) rather than be shut down, as was usual in the past. In this way, firstly, power can be fed into the power supply grid again as directly as possible at the end of the grid disturbance and, secondly, the power supply grid can be sustained, with respect to the voltage thereof, during the grid fault by feeding in reactive current. By way of example, a grid fault is present if the amplitude or rms (root mean square) value of a single-phase grid voltage is below a minimum value. In the case of power grids which supply multiphase power, an analogous definition can be given based on the average amplitudes of the individual phases, for example. Due to the significantly reduced grid voltage in such a case, only a small rms output voltage from the inverters is necessary to generate the required reactive and/or active current.
Inverters usually have at least one bridge arrangement via which an AC-voltage-side output of the inverter can be connected by means of semiconductor power switches to at least two different DC voltage potentials alternately. To control the level of the output voltage and to adjust the desired course of the output voltage, which shall be as sinusoidal as possible, a pulse width modulation (PWM) method is usually used. Good conversion efficiency can be achieved using so-called multilevel inverters, in which semiconductor power switches are arranged such that more than two different voltage levels are connectable to the AC-voltage-side output (multilevel modulation, for example three-level modulation). In order to actuate the semiconductor power switches of the bridge arrangement, which are at very different DC voltage potentials, actuation signals at likewise very different DC voltage potentials are correspondingly necessary. For this purpose, so-called bootstrap capacitors are often used on an actuation circuit, wherein the desired potential for the actuation of the semiconductor power switches is built up in the bootstrap capacitors. In this case, the bootstrap capacitors provided for supplying the power for the actuation of the semiconductor power switches that are at a high positive potential (high-side switches) are, in particular, charged during the on-time of the semiconductor power switches of the bridge arrangement that are at a lower potential (low-side switches).
However, during grid fault ride-through, the drive level, that is to say the ratio of the lengths of the periods when a semiconductor power switch is actuated to the periods when the semiconductor power switch is not actuated, can become so low that the bootstrap capacitors are not sufficiently recharged. As a result, the semiconductor power switches that are actuated by means of the bootstrap capacitors can no longer be switched. One solution to this problem can be achieved by using bootstrap capacitors with a correspondingly higher capacitance, so that activation can take place over the required period of the FRT even in the case of temporarily recharging the bootstrap capacitors only little or not at all. The use of bootstrap capacitors with a higher capacitance is not desirable, however, owing to the higher costs and the greater space requirement associated with such capacitors.